1. Field of the Invention
The invention relates to a driving force transmission apparatus that transmits driving force from an input shaft to an output shaft in, for example, an automobile.
2. Description of the Related Art
There is a conventional driving force transmission apparatus that is mounted in, for example, a four-wheel-drive vehicle and in which a pair of rotary members are coupled to each other by a clutch such that torque is transmittable therebetween (see, for example, Japanese Patent Application Publication No. 2008-121796 (JP 2008-121796 A)).
The driving force transmission apparatus is formed of a first rotary member, a second rotary member, a friction clutch and a pressing force application mechanism. The first rotary member rotates together with an input shaft. The second rotary member is rotatable about the axis of the first rotary member. The clutch couples the second rotary member and the first rotary member to each other such that torque is transmittable therebetween. The pressing force application mechanism applies pressing force, which is used as clutch actuating force, to the clutch.
The first rotary member is formed of a closed-end cylindrical housing that is open toward one side. The first rotary member is coupled to the input shaft. The first rotary member rotates upon reception of the driving force from a driving source, such as a vehicle engine, via the input shaft.
The second rotary member is arranged so as to be rotatable relative to the first rotary member, and is coupled to an output shaft. The second rotary member is accommodated in an apparatus case together with the first rotary member.
The clutch includes inner clutch plates and outer clutch plates, and is arranged between the first rotary member and the second rotary member. The clutch couples the first rotary member and the second rotary member to each other such that torque is transmittable therebetween, by frictionally engaging the inner clutch plates and the outer clutch plates with each other.
The pressing force application mechanism includes a piston and a pressing force application member. The piston is movable back and forth inside the apparatus case (cylinder). The pressing force application member applies the moving force of the piston as pressing force, to the clutch. The pressing force application mechanism is arranged on the input side of the clutch.
With the above-described configuration, when driving force from the engine is input into the first rotary member via the input shaft, the first rotary member rotates about its axis. At this time, if hydraulic fluid is supplied into the cylinder, pressure due to the hydraulic fluid acts on the piston.
Therefore, the pressing force application mechanism is actuated, the piston moves toward the clutch accordingly, and the moving force is applied from the pressing force application member to the clutch as pressing force.
Then, the inner clutch plates and the outer clutch plates of the clutch approach each other and then frictionally engage with each other, and the first rotary member and the second rotary member are coupled to each other due to the frictional engagement such that torque is transmittable therebetween. Thus, the driving force from the engine is transmitted from the input shaft to the output shaft via the driving force transmission apparatus.
With the driving force transmission apparatus described in JP 2008-121796 A, it is desirable to reduce the amount of lubricating oil (to lower the oil level) in the housing in order to reduce adverse influence due to so-called drag torque that is generated on the basis of the viscosity of the lubricating oil while the four-wheel-drive vehicle travels in a two-wheel-drive mode.
On the other hand, if the amount of lubricating oil in the housing is excessively small while the four-wheel-drive vehicle travels in a four-wheel-drive mode, there is a possibility that the clutch plates may be damaged due to heat of the clutch plates, which is generated through frictional engagement between the inner clutch plates and the outer clutch plates of the clutch, and the durability of the clutch may decrease.